1. Field of the Invention
The present invention relates to an image display medium, image display device, and image display method, and more particularly to an image display medium containing two types of particles whose colors and charging polarities are different from one another, and an image display device, and image display method using the medium.
2. Description of the Related Art
As image display media capable of displaying images, known as electronic paper, known examples include those making use of rotation of colored particles (twisting ball display), electrophoresis, magnetophoresis, thermal rewritable displays, image storable liquid crystals, electrochromy and the like. Of these image display media, the thermal rewritable display medium and image storable liquid crystals are excellent in storage of an image, but the background is not sufficiently white as in ordinary paper, the contrast between the image area and non-image area (background) is low, and it is difficult to display a sharp image. In particular, in a color display, only light of specific wavelengths is transmitted, and the quantity of light utilized in the image display is small as compared with the quantity of irradiated light.
When using color filters, three color filters, a red filter, green filter and blue filter, are often used as a set. One pixel electrode is disposed at each position opposite to one of the filters on a glass substrate. Since red, green and blue are the three primaries of additive color processes, all colors can be created by properly mixing these three primary colors. For example, when all of red, green and blue are mixed, white is created. The blue, green and red filters transmit visible light of wavelengths from about 400 nm to about 500 nm, from about 500 nm to about 600 nm, and from about 600 nm to about 700 nm, respectively, and absorb or reflect the remaining wavelength ranges of visible light. In other words, each color filter transmits only about ⅓ of the light in the range 400 nm to 700 nm, and the light in the remaining ⅔ of the range is not utilized. Accordingly, when each color filter is irradiated with white light, in principle, the transmissivity of the color filter is about 30%.
The actual transmissivity of a polarizer or a color filter is lower than the theoretical value, and further, when the image display medium has an active element such as a TFT, the utilization efficiency of light is lowered by its aperture ratio. Accordingly, when the image display medium is of a reflective type, it is difficult to achieve a final utilization efficiency, that is, reflectivity, of 10% or more unless a special measure is applied to the light reflective layer or the like. The current target of brightness or whiteness in an image display medium is equivalent to that of a newspaper, so a reflectivity of about 60% is required, but since this target value is far beyond the theoretical limit in the image display medium of the above-described system, evidently it is required to change the method radically, including omission of a polarizer, in order to achieve the target.
In a display medium utilizing electrophoresis or magnetophoresis without using a polarizer, colored particles that are movable by applying an electric field or magnetic field are dispersed in a white liquid, and an image is formed by, for example, adhering colored particles to the display surface to display the color of colored particles in the image area, and removing the colored particles from the display surface in the non-image area to display the white color of the white liquid. Since the colored particles cannot be moved without action of an electric field or magnetic field, the image can be stored in this display medium. In such display media, the background maybe displayed in a clean white, but a sufficient image density cannot be obtained in the image area, because the white liquid enters gaps between the colored particles. As a result, sufficient contrast cannot be obtained between the image area and non-image area, and it is difficult to display a sharp image. Further, if the display medium is bent when the medium is removed from an image display device, the white liquid may leak out from the display medium.
The twisting ball display is designed to display an image by inverting spherical particles, half the surface of the particles being colored white and the other half being colored black, by the action of an electric field. For example, an electric field is applied thereto such that the black sides orient toward the display side in the image area and the white sides orient toward the display side in the non-image area. The particles in this display medium are not inverted unless an electric field is applied thereto, so the image can be stored. Inside the display medium, oil is present only in the cavities around the particles, and since almost only solid material is contained in the medium, it is relatively easy to form the display medium into a sheet form. In this display medium, if an electric field is applied to the display medium so that the entire display area is white, rays of the light incident upon the display medium that stray into gaps between the balls are not reflected but are lost in the interior of the medium. Accordingly, in principle, 100% white cannot be displayed. Moreover, because of light absorption or light scattering in the cavities, only a grayish white color is displayed. Further, it is difficult to invert the particles completely, which results in a reduction of contrast and it is hence difficult to display a sharp image. In addition, since the particle size is required to be smaller than the pixel size, fine particles painted in different colors must be prepared for displaying an image of high resolution, and sophisticated manufacturing techniques are required. Thus, in these image display media, the display performance of at least one of white and black is low, and the color saturation is low when a color image is displayed using such media. Moreover, these image display media are made of materials which have poor light fastness, and which are apt to degrade at high temperatures, and they cannot be used outdoors. In addition, when the media are left standing in automobile compartments at high temperatures, there have been problems such that images cannot be displayed.
Further, as an image display medium with a white background, it has been proposed to use a medium in which electro-conductive color toner and white particles are enclosed between a pair of opposing substrates. In this image display medium, electric charge is injected into the electro-conductive color toner through a charge transfer layer provided on the inner surface of an electrode of a back substrate, and the electro-conductive toner to which the charge is injected is moved to the display substrate by the electric field between the electrode substrates, and is adhered to the inner surface of the display substrate to display an image (“Toner Display”, Japan Imaging Society, Japan Hardcopy '99 papers, pp. 249-252; Japan Hardcopy '99 fall papers, pp. 10-13). This image display medium has advantages that the light fastness of the medium is high in principle, the medium can be driven even if left in high temperatures, and the medium in principle can change over the pixel colors completely. In this image display medium, however, there are electro-conductive color toner particles which do not contact the charge transfer layer and electro-conductive color toner particles which are isolated from other electro-conductive color toner particles. Electric charge is not injected into these electro-conductive color toner particles, and they are not movable by an electric field. Accordingly, these color toner particles are located randomly between the substrates, and contrast cannot sufficiently be enhanced.
The present inventors have proposed an image display medium comprising a pair of substrates and a plurality of types of particle groups, whose colors and charging characteristics are different from one another, enclosed between these substrates (Japanese Patent Application No. 2000-165138). In this display medium still, since the particles are not moved if an electric field is not applied to the particles, the image can be stored. Further, since the entire image display medium is composed of solid substances, liquid does not leak. However, if one attempts to display a multi-color image by using this image display medium, black color must be displayed as a mixed color of chromatic color by using chromatic colors particles and white particles, and therefore a black image having a sufficient density cannot be obtained.